Solution synthesis and optical tunability of low toxicity semiconductor nanocrystals

Abstract

Semiconductor nanocrystals have the potential to revolutionize several important applications inducing medical imaging and solar cells. The most popular semiconductor nanocrystals are cadmium or lead chalcogenides due to their ease of synthesis, well studied and superior optical properties. Both cadmium and lead are toxic metals and therefore for the practical utilization of semiconductor nanocrystals materials of lower toxicity are required. The theme of this thesis was to develop the (synthetic and optical) prospects of low toxic semiconductor nanocrystals for applications in medical imaging or solar cells. Two families of semiconductors with differing applications were explored, group IV (silicon and germanium) for bioimaging applications and copper chalcogenides for solar cells. Chapters 1 and 2 provide context to the thesis and experimental methods used. Chapters 3, 4, and 5 focuses on the synthesis and bioimaging applications of silicon and germanium nanocrystals. Chapter 6 focuses on the synthesis of quaternary copper chalcogenide Cu2ZnSnS4. Chapter 7 covers general conclusions of the thesis and future directions. Chapter 3 describes the development of a generalized method to produce metal doped silicon nanocrystals. The role of dopant identity and concentration on the optical properties are investigated. It was found that metal dopants induce unique changes to the optical properties (emission and excitation redshifts, enhanced quantum yields). Chapter 4 builds on the work of chapter 3 by showing that the enhanced optical properties of metal doped silicon nanocrystals lead to bioimaging improvements by the complete in vitro reduction of autofluorescence. Cytotoxicity tests also demonstrate that metal doping does not alter the toxicity of silicon nanocrystals. Chapter 5 looks at developing methods to tune the optical properties of germanium nanocrystals. The effect of alloy formation and surface capping molecules are examined. Alloy formation proved difficult with the current synthetic methodology used. Surface capping molecules were found to strongly alter the emissive properties of germanium nanocrystals. Chapter 6 looks at developing a seed mediated approach to the synthesis of Cu2ZnSnS4 nanocrystals. Seed mediated approach allows the production of Cu2ZnSnS4 nanocrystals with good control over key synthetic variables. Cu2ZnSnS4 nanocrystal size and composition could be altered by simple changes to reaction time and precursor equivalents.